The technical part of the profession of an optician consists in mounting a pair of ophthalmic lenses in an eyeglass frame that has been selected by a client, and it comprises four main operations:                acquiring the shapes of the outlines of the surrounds of the eyeglass frame selected by the client;        centering each ophthalmic lens in a centering and blocking appliance, which consists in identifying the frame of reference of the lens with the help of centering marks provided thereon, and then appropriately positioning the ophthalmic lens in the appliance;        blocking each lens, which consists in fastening a blocking accessory on the lens in such a manner as to enable the lens subsequently to be gripped and moved without losing its frame of reference; and then        shaping each lens, which consists in machining the lens to have the outline as acquired in a frame of reference that is identified relative to the blocking accessory, and in such a manner that once the lens has been mounted in the eyeglass frame, it is correctly positioned relative to the corresponding eye of the client so as to perform, as well as possible, the optical function for which it has been designed.        
More precisely, the present invention relates to the blocking operation which, in practice, consists in placing the blocking accessory on the ophthalmic lens in such a manner as to enable it to adhere to the front face of the lens in releasable manner.
It is possible for the blocking accessory to be put into place manually. However that is found to be relatively inaccurate, such that in practice this is usually done with the help of a blocking device.
A blocking device of the kind defined in the introduction is well known from document FR 2 608 492.
In that document, the manipulation arm is of elongate shape, having a first end that carries the gripper means for gripping the blocking accessory, and a second end that carries a cam with a portion that is rectilinear and another portion that is rounded.
The manipulation arm is then mounted to move between two extreme positions referred to as a rest position and as an activation position.
In the rest position, the manipulation arm extends horizontally at a distance from the lens support, so that the optician can easily access the gripper means in order to install a blocking accessory thereon.
In the activation position, which marks the end of a rectilinear portion of the path of the manipulation arm, the blocking accessory comes into contact with the ophthalmic lens that is to be fitted therewith.
The path of the manipulation arm between these two extreme positions is guided by the cam in that example.
Movement is imparted to the arm by two units fitted with two respective facing rectilinear racks that are driven by a common gearwheel so that both of them move in translation parallel to each other but in mutually opposite directions.
The drawback of that device is that it is bulky, that it presents architecture that is complex, and that it is therefore expensive to fabricate.